When I try to compile the following code I get the error C2280. I guess the compiler is trying to copy the unique_ptr or something.
#include <memory>
std::pair<int, std::unique_ptr<int>> CreatePair()
{
std::unique_ptr<int> my_int(new int);
return { 1, std::move(my_int) };
}
std::unique_ptr<int> GetUinquePtr()
{
auto [ignore, unique_ptr] = CreatePair();
return unique_ptr; // <- Build error C2280 attempting to reference a deleted function
}
int main()
{
auto unique_ptr = GetUinquePtr();
}
Complete error message:
error C2280: 'std::unique_ptr<int,std::default_delete<int>>::unique_ptr(const std::unique_ptr<int,std::default_delete<int>> &)': attempting to reference a deleted function
It works if I add std::move():
std::unique_ptr<int> GetUinquePtr()
{
auto [ignore, unique_ptr] = CreatePair();
return std::move(unique_ptr); // <- This works
}
And it works fine if I use std::tie:
std::unique_ptr<int> GetUinquePtr()
{
std::unique_ptr<int> unique_ptr;
std::tie(std::ignore, unique_ptr) = CreatePair();
return unique_ptr; // <- This works
}
So do need to explicitly type std::move after structured binding of a unique_ptr or am I doing something wrong here?
A structured binding creates references, and your code is more or less equivalent to this:
std::unique_ptr<int> GetUinquePtr()
{
auto p = CreatePair();
auto& ignore = p.first;
auto& unique_ptr = p.second;
return unique_ptr;
}
and returning the reference would create a copy.
With the tie
, it works rather like this instead:
std::unique_ptr<int> GetUinquePtr()
{
std::unique_ptr<int> unique_ptr;
unique_ptr = CreatePair().second;
return unique_ptr;
}